100 Player Servers are servers which can handle up to 100 players, while an average server can handle 50 players. Servers can become very slow at about 80 players on weak connection, but more efficient games or faster internet speeds may get better results. To obtain them, you have to be on the beta list (with BC). Currently, only well-known developers are known to have them, but you can email Roblox support to obtain a 100 player server On June 18th, 2018, a roadmap was released to showcase what the Roblox team is developing or planning to develop. 100 player servers were mentioned and are targeted for development and/or release during the 2nd quarter of the roadmap (May - June).
Somewhere in early 2019, Roblox changed the maximum players for all games to 100.